Aquarium Water Change Calculator
Usable litres in a tank and the volume of a partial water change, with the dose of conditioner needed.
Results
What this tool does
The number printed on an aquarium is the glass volume, and it is never the volume of water in it. Substrate, hardscape and the gap below the rim all take their share, and everything that gets dosed — dechlorinator, fertiliser, medication — is measured against the water that is really there. A regular partial change of a quarter is the routine most freshwater tanks are kept on.
Formula
volume = L × W × H ; usable volume = volume × (1 − displacement) ; change = usable volume × percentage
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ll | Length | cm |
ww | Width | cm |
hh | Height | cm |
ds | Substrate and decor | % |
pc | Share of water changed | % |
dr | Conditioner dose | mL/50 L |
NV | Actual water volume | L |
GV | Tank volume | L |
CV | Water to change | L |
CD | Conditioner to add | mL |
KG | Weight | kg |
Worked example
- Length100 cm
- Width40 cm
- Height50 cm
- Substrate and decor10 %
- Share of water changed25 %
- Conditioner dose5 mL/50 L
- Actual water volume180.0 L
- Tank volume200.0 L
- Water to change45.0 L
- Conditioner to add4.50 mL
- Weight180.0 kg
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
Frequently asked questions
Why is the usable volume less than the tank volume?
Three things take up room: the substrate at the bottom, the rocks, wood and plants inside, and the few centimetres of air deliberately left below the rim. Together they typically account for a tenth to a fifth of the nominal volume, which matters because every dose of medication or conditioner is calculated on the water that is actually there, not on the size printed on the box.